This invention relates to the field of intravascular catheters, and more particularly to an inflatable member formed in part of liquid crystal polymeric material.
Balloon catheters generally comprise a catheter shaft with a inflatable member on the distal end of the shaft, and are used in a number of procedures, such as percutaneous transluminal coronary angioplasty (PTCA). In PTCA the balloon catheter is used to restore free flow in a clogged coronary vessel. The catheter is maneuvered through the patient""s tortuous anatomy and into the patient""s coronary anatomy until the inflatable member is properly positioned across the stenosis to be dilated. Once properly positioned, the inflatable member is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. greater than 4 atm) to reopen the coronary passageway.
The material used to make the catheter inflatable member must have sufficient strength to contain the inflation fluid without bursting. In addition, the degree of compliance must be tailored so that the inflatable member expands during use, but does not overexpand and damage the body lumen. During extrusion and subsequent processing of polymeric inflatable member tubing, the longitudinal and radial orientation of the polymeric molecules can be tailored to increase the longitudinal and radial strength of the inflatable member produced therefrom. Because the force required to blow a inflatable member from tubing destroys some of the longitudinal orientation produced during extrusion, the extrusion process, and particularly the draw down ratio, is designed around optimizing the molecular orientation that is ultimately produced in the finished inflatable member. The strength of a inflatable member is typically expressed in terms of hoop strength and burst pressure.
Inflatable members formed from thermoplastics such as PET blended with liquid crystal polymers to improve the compliance of the inflatable member have been suggested (U.S. Pat. No. 5,306,246 (Sahatjian et al.)). However, Sahatjian et al. fails to address the problem of lower burst pressures of inflatable members produced from blends of a minor amount of liquid crystal polymer with a major amount of a non-liquid crystal polymer, relative to inflatable members produced from only thermoplastics such as PET.
Therefore, what has been needed is a catheter with an inflatable member having improved strength characteristics. The present invention satisfies these and other needs.
The invention is directed to a catheter which has an inflatable member formed of a polymeric blend comprising a minor amount of a liquid crystal polymeric material and a major amount of a non-liquid crystal polymeric material. There are many suitable liquid crystal polymeric materials that may be used, and a presently preferred example is VECTRA sold by Hoechst-Celanese. The non-liquid crystal polymeric material used in the LCP blend may be any extrudable thermoplastic polymer, and presently preferred examples are nylon 12 available from EMS, and PEBAX available from Atochem.
The LCP acts as reinforcement in the polymer matrix, similar to fiber reinforced composites, but at significantly lower dimensional scale and ease of processing. One presently preferred embodiment of the invention is a dilatation catheter which has an elongated catheter shaft and an inflatable dilatation member on a distal portion of the catheter. Conventional catheter design may be used, including over the wire, fixed wire and rapid exchange designs, having a single shaft with dual lumens or a multimembered shaft with inner and outer tubular members.
Liquid crystal polymers exhibit crystalline behavior in the liquid phase. The orientation of the molecules in the liquid state can be maintained in the solid state due to the long relaxation times of these molecules. The molecular orientation improves the strength of a polymeric component in the direction of orientation. The extent of molecular orientation can be expressed in terms of the Herman""s orientation parameter (S) with a scale of from 0 (no orientation) to 1 (very highly oriented), which is a factor of the draw-down ratio (i.e. the ratio of the diameter of the die to the diameter of the finished extrudate) used during extrusion and viscosity. Liquid crystal polymers can be made to solidify after extrusion with an even greater degree of molecular orientation than ordinary polymers, and thus can be used to form ultra high strength articles. However, the high molecular orientation may result in disadvantageous characteristics in an inflatable member, such as little ability to withstand loads applied transverse to the orientation direction, increased stiffness, and poor bonding between layers. These disadvantages are avoided by the polymer blend of the invention. In the polymer blend of the invention, the Herman""s orientation parameter is about 0.5 or less. Thus, the inflatable member of the invention formed from a minor amount of liquid crystal polymeric material existing as elongated liquid crystal polymeric fibers having an aspect ratio of about 10 to about 100, and preferably about 50 to about 100 has improved strength characteristics.
The inflatable member is formed from a blend comprising a minor amount, preferably less than about 20 to about 10 percent by weight of the blend, of liquid crystal polymeric material, with a major amount, preferably about 80 to about 90 percent by weight of the blend, of a non-liquid crystal polymeric material (hereafter, the LCP polymeric blend). By blending the liquid crystal polymeric material with another thermoplastic material the transverse strength of the inflatable member produced therefrom is increased. Additionally, it has been found that an inflatable member having LCP fibers that are highly oriented in the machine direction has improved mechanical characteristics. Specifically, if the aspect ratio of the liquid crystal polymer fibers is greater than 100, the LCP polymeric blend has mechanical characteristics similar to a continuous long-fiber-reinforced composite. As a result, the main load on the inflatable member is taken by the fibers (iso-strain process), as opposed to short fiber or dispersed phase composites where the load is shared by the filler and the matrix (iso-stress). Therefore, the LCP polymeric blend having a high aspect ratio can be used to produce thin walled inflatable members with improved burst pressures. Additionally, the risk of inflatable member overexpansion during use is reduced, because the high aspect ratio results in an advantageous reduction in inflatable member compliance.
An inflatable member formed from the LCP polymeric blend of the invention has improved strength and optimal compliance due to the high aspect ratio produced during extrusion. These and other advantages of the invention will become more apparent from the following detailed description of the invention and the accompanying exemplary drawings.